1. Field of the Invention
The present invention is directed generally to gate valves for pipe lines and the like and more specifically to expanding gate valves with relatively movable gate and segment assemblies having oppositely inclined interacting cam surfaces that cause expansion thereof for tight sealing with internal sealing surfaces with the gate valve and permit contraction thereof to permit opening and closing movement of the gate and segment assemblies by a linearly movable valve stem. Even more specifically, the present invention concerns a compact expanding gate valve mechanism having a gate and segment assembly incorporating control arms which are connected to the gate and segment actuating pins for achieving desired positioning of the gate and segment at various positions during opening and closing movement of the valve mechanism.
2. Description of the Prior Art
Expanding gate valves have been in wide use for many years and achieve efficient sealing particularly at both the open and closed positions of the valve mechanism. Typically, within a valve body structure a gate member is movable by a linearly movable valve stem between an open position, where flow of fluid through inlet and outlet passages is permitted, and a closed position where the gate member blocks the flow of fluid through the inlet and outlet passages. The gate member may define a flow port which is in registry with the flow passages in the open position of the gate and segment assembly. In compact gate valves, the gate member is often non-ported and is moved by the actuating stem of the valve to an open position where it is retracted to a position within the bonnet structure of the valve body and substantially clear of the flow passages.
Efficient sealing of the gate and segment assembly with respect to internal seat rings or seat surfaces located within the valve body about the flow passages is caused by one or more segment members which cooperate with the gate member to define a gate and segment assembly and which is positionable relative to the gate member to permit contraction of the gate and segment assembly to thus permit its movement between the open and closed positions of the valve mechanism. This expansion and contraction of the gate and segment assembly is caused by interacting wedge or cam surfaces of the gate member and the segment member is caused by interacting cam or wedge surfaces of the gate and segment members which typically occurs as the gate member is driven linearly by the valve stem while the segment member is restrained by suitable internal stop structure of the valve body.
A problem that typically occurs when expanding gate valves are employed is the tendency of the gate and segment assembly to remain in its condition of expansion even when force is being applied by the valve stem to achieve its contraction and movement. To minimize the tendency of the gate and segment assembly to remain so expanded, springs are often used to provide an urging force for collapsing or contracting the gate and segment assembly in absence of the presence of cam induced forces that overcome the spring force and expand the gate and segment assembly or structure that prevents collapse of the gate and segment assembly and maintains it positioned at its open or closed positions.
Another problem that often occurs is the tendency of conventional gate and segment assemblies to undergo a condition known as "back-wedging" wherein the gate and segment assembly will be expanded particularly during its movement toward the closed position responsive to pressure induced force. As the gate and segment assembly are moved typically downwardly toward the closed position, as the flow path through the valve becomes restricted the velocity of the flowing fluid will increase. This increased velocity flow will act on the segment member, forcing it tightly against the downstream seat and thus increasing its drag or frictional resistance to movement. When this condition is developed the gate member is physically moved by the valve stem while the segment tends to become static at an intermediate position. This condition results in relative movement of the gate and segment members, thus causing expansion of the gate and segment assembly during its travel between the open and closed positions.
Resistance to opening and closing movement of the gate and segment assembly can also result from increased pressure within the valve chamber as compared to pressure within the flowway of the valve. This higher body pressure can develop as the result of temperature changes, such as when fluid trapped within the valve body externally of the seat rings can expand as the result of increased heat and cause body pressure of increase. This increased body pressure can act on the gate and segment assembly and enhance its frictional engagement with the valve seats to the point that opening movement of the gate and segment assembly requires excessive valve stem force. It is desirable therefore to ensure that the gate and segment assembly does not become expanded by back-wedging during its movement between the open and closed positions. It is also desirable that the gate and segment assembly become collapsed immediately as its movement toward the open or closed position is initiated and remain collapsed during the entirety of its travel between the open and closed positions.